T cells bearing T cell receptors (TCRs) are responsible for driving specific immune responses against invading organisms and, in the case of autoimmunity, against self. The TCRs on these cells react with antigenic peptides bound to major histocompatibility complex proteins (MHC). The reasons for the bias of TCRs for interaction with MHC has long been debated, however, recent evidence suggests that evolution has selected for amino acids at certain positions on TCRs that have a built in likelihood of engaging MHC. This project will investigate the nature of the sites on MHC that are the reciprocal of those on TCRs, i.e. that consistently engage the evolutionarily selected MHC-reacting amino acids of TCRs. Although evidence suggests that TCRs have the ability to react generically with MHC, individual TCRs are certainly specific for particular MHC proteins, as witnessed by the MHC allele restriction conferred on T cells during positive selection in the thymus. This project will study the structural bases for MHC allele specificity on the part of T cells. Regulatory T cells prevent immune responses against certain tissues in the body. It is thought that they do this by reacting with MHC bound to self peptides that are present in the tissue at issue. In spite of much work, little is known about the endogenous peptides that are recognized in normal mice by endogenous regulatory T cells. Mice and methods developed in this Project will be used, in conjunction with MHC/peptide libraries, to identify self peptides tha are recognized by regulatory T cells in normal mice. Overall, this Project will investigate various aspects of the TCR/MHC/peptide interaction. It will thus provide a firmer basis for our understanding of the structural bases for TCR interaction with generic and particular MHC proteins and illuminate the long mysterious process of T cell positive selection. Studies will also establish the nature of self peptides recognized in association with MHC by regulatory T cells, a discovery which will help understanding of the mode of action of regulatory T cells and perhaps improve ability to manipulate these cells in vivo.